Esters derived from polymeric fatty acids obtained by the dimerization of unsaturated fatty acids are known and widely used as synthetic lubricants. Products of this type are described in U.S. Pat. Nos. 2,767,144 and 3,223,635. Even though esters and polyesters having excellent lubricating properties are obtained from the reaction product of dimer acid and glycols, the compatibility of these products with water is generally poor. This is of little consequence if the lubricant is to be employed with a petroleum or synthetic lubricating base stock, however, for many metal working applications it is highly desirable to utilize the lubricant in an aqueous system.
Dimer-based synthetic ester lubricants having increased hydrophilic character, i.e. which exhibit a marked affinity for water, have been developed. U.S. Pat. No. 3,492,232, for example, discloses ester lubricants derived from polyoxyalkylene glycol and a polymeric fatty acid. While these lubricants can be employed as aqueous metal working lubricants and represent an advance in the art, they are not without certain disadvantages. Aqueous emulsions of these products are not completely stable and undergo partial phase separation in short periods of time. If stable emulsions are to be obtained, the use of external emulsifying agents is necessary.
Further improved lubricants derived from polyoxyalkylene glycols and polymeric fatty acids are disclosed in U.S. Pat. No. 3,769,215. The emulsifiability of the synthetic lubricant product and stability of the resulting aqueous emulsions is enhanced by incorporating a short-chain dibasic acid in the ester. It is thus possible to form stable aqueous emulsions of these synthetic ester lubricants without the use of external emulsifying aids and, even more unexpectedly, some of these ester lubricants are completely soluble in water so that clear, homogeneous lubricant solutions can be prepared. While the ester lubricants of U.S. Pat. No. 3,769,215 and aqueous emulsions and solutions thereof are extremely useful and exhibit a high degree of lubricity, they have little or no rust preventive ability. These problems can be overcome by the additives of suitable additives but not without some sacrifice in the other desirable properties of the system and additional cost to the formulator.
It would be highly desirable, therefore, if dimer-based synthetic lubricants exhibiting rust preventive character and enhanced lubrication properties, particularly improved extreme pressure properties, were available. It would be even more advantageous if these dimer-based lubricant compositions could be obtained without adversly affecting the hydrophilic character of the polymer so that stable aqueous lubricant systems could be prepared. These and other advantages are possible with the improved lubricant compositions of this invention.